(a) Field of the Invention
The present invention relates to a rear converter lens system which is interposed and removed from between a photographic lens system (master lens system) and the image surface formed by said photographic lens system for varying focal length of a photographic optical system.
(b) Description of the Prior Art
In the field of the optical instruments such as cameras, it is known that focal length of a photographic optical system is varied by attaching a converter lens system to a photographic lens system. Since the rear converter lens system which is designed for interposition between the master lens system and the image surface thereof has strongly negative refractive power to obtain a focal length longer than that if the master lens system, the optical system as a whole has a large negative Petzval's sum when the rear converter lens system is attached to the master lens system. In order to prevent aberrations from being varied in the optical system and maintain at a small value the Petzval's sum of the optical system which is apt to be enlarged, the conventional rear converter lens system is composed mostly of approximately five lens components, and has defects that it is large in dimensions and that it requires high manufacturing cost.
As rear converter lens systems developed for correcting these defects, there are known the lens systems disclosed by Japanese Examined Published Patent Application No. 45207/61 and Japanese Unexamined Published Patent Application No. 148222/63. Each of these lens systems is composed of two lens components, i.e., a positive lens component and a negative lens component.
However, each of these lens systems has a large negative Petzval's sum and aberrations which are not corrected sufficiently.